Field of the Invention
The present invention relates to a process for regeneration of oxidative dehydrogenation (OXO-D) catalyst in a reactor. More particularly, the present invention relates to a process for regeneration of iron-based oxide catalyst in a reactor.
Description of the Related Art
Butadiene, commonly known as 1,3-butadiene, is used as a monomer in the production of synthetic rubber. Butadiene is a basic petrochemical raw material used for several industrial applications including tire and polymer industry. Although, there are numerous methods to produce and obtain butadiene, oxidative dehydrogenation (OXO-D) reaction of butene is the most efficient process to produce butadiene. Butadiene is obtained by oxidative dehydrogenation of n-butenes (1-butene and/or 2-butene) and any mixture comprising n-butenes can be used as starting gas mixture.
The use of multi-metal oxide catalyst in OXO-D reaction enhances the productivity, selectivity and conversion ratio of butadiene. A wide range of catalysts such as oxides of molybdenum, vanadium, cobalt, zinc, and the like, have been used to alter OXO-D reaction. However, most of these catalysts exhibit problems such as low selectivity, stringent operating conditions, high energy consumption and the like. Of all the catalysts, iron based oxide catalysts were identified to have advantages, such as higher butadiene yield and lower oxidation byproducts. Therefore, iron based oxide catalysts are most widely used in the oxidative dehydrogenation (OXO-D) reaction.
In the oxidative dehydrogenation of n-butenes to butadiene, a carbonaceous material can be formed on the multi-metal oxide catalyst, and which can ultimately lead to its deactivation. The multi-metal oxide catalyst is regenerated by burning off the deposited carbon at regular intervals by means of an oxygen-comprising gas in order to restore the activity of the catalyst. However, there is wide difference in the operating and regenerating conditions of OXO-D reaction and the catalyst, respectively. For example, temperature required for regeneration of the catalyst is higher than temperature required for operating conditions of the OXO-D reaction. In order to sustain the variation in temperatures over a period of time, the reactor has to adjust frequently to re-equilibrate thereby leading to inefficient usage of the reactor. Moreover, the temperatures above reaction conditions wear the reactor and also burn excess saturated carbonaceous compounds, which may also result in charring and destruction of the catalyst.
Relative to regeneration of the catalyst, much is written about regeneration of catalyst for an OXO-D process wherein the reactor is a fluidized bed or a single reactor is employed in-situ. Also, much of what is written is directed towards Mo Bi catalyst and not as much to ZnFe catalyst where it has been found that regeneration conditions are critical to avoid charring. References do not appear to take into consideration the sensitivity of the ZnFe based oxide catalyst to temperature and the need to protect the catalyst during the regeneration process.
U.S. Pat. No. 3,595,809 to Kehl discloses regeneration of a lanthanum chromium ferrite catalyst composition wherein the catalyst is calcined in air at 500° C. to 650° C.
U.S. Pat. No. 3,595,810 to Kehl discloses a zinc chromium ferrite catalyst of a spinel structure having a distinct crystalline structure which can be regenerated in a single reactor. Regeneration occurs in air at about 500-650 C. See col 4, example 1, however, note that chromium makes a catalyst behave differently than a zinc ferrite due to its different structural, surficial and chemical interactions or bonds.
U.S. Pat. No. 3,669,877 to Friedrich discusses a multi-chamber fluidized bed catalytic reactor where the ferrite catalyst comprising crystalline composition of iron, oxygen and other metals are regenerated in the same reactor as the OXO-D reaction of the n-butene to butadiene.
U.S. Pat. No. 4,044,067 to Besozzi et al. discloses a purification of unsaturated compounds, or removal of oxygenated compounds on the preferred ZnFe catalyst, by passage of air or steam to remove coke deposits on the catalyst. Minimal details are provided regarding the regeneration process (see col 6 1 28).
US 2010/0248942 to China Petroleum and Chemical Corporation discusses regeneration of catalyst for improving performance and selectivity in a cracking process involving transfer of the catalyst to a separate regeneration reactor, and the regeneration times are less than 30 minutes.
US 2012/0164048 to Duff et al disclose a zinc-free catalyst system and a process for the selective removal of acetylenic compounds from a butadiene production stream. Para 0024 discloses that the catalyst can be regenerated by controlled oxidation with or without steam in the absence of hydrocarbons. The process disclosed is generic with no parameters provided for the actual regeneration conditions.
US 2014/0163292 to Gruene et al disclose a process for the oxidative dehydrogenation of n-butenes to butadiene and a regeneration step of the molybdenum cobalt catalyst employed. However, while it is disclosed that 5 cycles of production and regeneration are employed, minimal details are provided for the regeneration step. It appears the regeneration occurs in a 15-30 minute process of passing an oxygen/nitrogen/water mixture over the catalyst.
Various Chinese publications disclose regeneration of an OXO-D iron based oxide catalyst, but do not provide details regarding the process:
CN 103071430 discusses radial fixed bed reactor for production of butadiene using a ferrite catalyst, wherein a regeneration process was employed at a cycle of 3, 9, 10, 12, and 14 months.
CN 103071544 discusses an in-situ regeneration method for OXO-D catalyst of ZnFe and MoBi.
CN 104226334 discusses regeneration of an OXO-D catalyst for production of butadiene wherein the catalyst is based on ZeFe or MoBi spinel composite oxide structure. An oxygen/steam mixture is passed over the catalyst. The regeneration method discusses a two stage process where oxygen/steam mixture is passed over the catalyst at 460° C. for 24-72 hours, to regenerate the catalyst efficiently. However, the production and regeneration take place in a single reactor and the reactor is exposed to extreme temperatures over a period of time, thereby leading to wear and less productivity of the reactor.
WO 2014/086815 discusses a method for the oxidative dehydrogenation of n-butene to butadiene. The reference discloses a regeneration step and the use of molybdenum catalyst in a fixed bed reactor. The regeneration occurs between production steps and utilizes oxygen gas passed over the catalyst at a temperature of at least 350° C. and at least 50° C. above the temperature of the prior production step. This requires the reactor be heated for the regeneration step, and then subsequently cooled for the following production step.
There is a need to develop a method, which provides successful regeneration of the catalyst at lower temperatures. Therefore, a need exists for a process which regenerates a catalyst and yet maintains the efficiency and productivity of the reactor. Also, there exists a need to utilize a multi-stage reactor with a spare reactor to regenerate catalyst off-line, in order to reduce non-production times.